N-Acyl amino acid salts are anionic surfactants useful in laundry detergents, household or industrial cleaners, foamers, emulsifiers, personal cleansers, and other applications. Because they are often exceptionally mild, the salts are particularly valuable for personal care formulations.
In general, N-acyl amino acid salts have been underutilized, due at least in part to challenges in manufacturing them. N-Acyl amino acid salts can be made from the corresponding fatty acyl chlorides and amino acid salts using Schotten-Baumann chemistry (see, e.g., J. Am. Chem. Soc. 78 (1956) 172 and U.S. Pat. No. 6,703,517), but this process is expensive and generates an equimolar amount of undesirable salt by-product. In an alternative synthetic method, a fatty acid is reacted with an amino alcohol to give a fatty amide, which is then oxidized to give the N-acyl amino acid (see, e.g., U.S. Pat. No. 8,093,414). This process is hampered by relatively low yields, low selectivities in the oxidation step, the use of precious metal catalysts, and the need for a conventional organic workup.
In other known processes, the N-acyl amino acid salt is made from a fatty acid. For example, EP 1672055 and U.S. Pat. Appl. Publ. No. 2006/0239952 describe the synthesis of N-acyl glycinates by reacting a fatty acid with glycine. This process generates a relatively high proportion of di- and tripeptide by-products (di- and triglycinates), which may or may not be desirable depending upon the intended use; conversion to the mono-acylated product is about 92%. U.S. Pat. No. 3,836,551 teaches to react fatty acids with amino acid salts either in the molten fluid phase (i.e., without a solvent), in solution using a polar aprotic solvent (such as dimethyl sulfoxide or N,N-dimethylformamide), or in suspension with a nonpolar organic solvent (e.g., xylene). Typical reaction times are about 9 hours, and by-products are not discussed. Generally, the fatty acid route is also less preferred because it requires a high reaction temperature, which leads to undesirable color development in the N-acyl amino acid salt.
U.S. Pat. No. 5,898,084 describes the preparation of N-acyl amino acid salts by reacting a mono-, di-, or triglyceride with an amino acid salt in the presence of a strong base. In the examples, colza oil (a triglyceride) is reacted with sodium sarcosinate in the presence of sodium methoxide/methanol, and the reaction continues until glycerides are no longer detected. A typical organic workup follows. The reference indicates that the glycerin produced in the course of the reaction either remains in the reaction mixture or is partly or wholly removed in the conventional workup. At the conclusion of the reaction, the mixture is typically a viscous paste.
Fatty alkyl esters have also been used as starting materials. U.S. Pat. No. 5,856,538 teaches to react a fatty alkyl ester (e.g., methyl oleate) with an amino acid salt and a 30-150% molar excess of a strong base, e.g., sodium methoxide/methanol solution (see col. 2, I. 65 to col. 3, I. 2 and Examples 2 and 3). Sodium sarcosinate is used in the examples, although other amino acid salts are taught as suitable. The '538 patent teaches (col. 3, II. 22-27) that the reaction is normally carried out “under atmospheric pressure; although autogenous pressure or elevated pressure is possible, it has no further advantages.” WO 95/07881 teaches a method of preparing N-acyl sarcosinates starting from fatty esters. The reference indicates that alcohol solvents (e.g., 1-propanol, 1-butanol, isobutyl alcohol, propylene glycol, ethylene glycol) can be used to reduce viscosity during the amidation reaction. In the examples, the solvent is used to remove water by azeotropic distillation.
Recent Unilever publications describe the preparation of fatty N-acyl amido surfactants from fatty alkyl esters and amino acid salts in a low-molecular-weight polyol such as glycerol or propylene glycol as a reaction medium (see U.S. Pat. Appl. Publ. Nos. 2013/0029899, 2013/0030197, 2013/0030198, 2013/0030199, 2013/0030200, 2013/0030201, 2013/0030202, and 2013/0030203). Comparative runs using water, alcohols, or toluene instead of glycerol provided very low yields. Catalysts taught for the process include “alkaline and alkaline earth metal containing hydroxides, phosphates, sulphates and oxides including calcium oxide, magnesium oxide, barium oxide, sodium oxide, potassium oxide, calcium hydroxide, magnesium phosphate and mixtures thereof” (see the '203 publication at paragraph [0031]). Calcium oxide is used in the examples.
The preparation of N-acyl amino acid salts is particularly challenging when the reactants are fatty alkyl esters (particularly fatty methyl esters) and alkali metal glycinates, as in the preparation of sodium cocoyl glycinate, sodium myristyl glycinate, or sodium lauryl glycinate. This reaction is troublesome due to a lack of reagent compatibility, solidification of the reaction mixture at elevated process temperatures, color development, severe foaming during methanol removal, and significant by-product generation. Too often, conditions cannot be found that provide low-color N-acyl amino acid salts in high yield with minimal generation of fatty acid soaps and dipeptide by-products.
In sum, an improved process for making N-acyl amino acid salts is needed. In particular, the industry needs a process that avoids salt generation and the selectivity issues of other known routes. Preferably, the particular difficulties that complicate the preparation of N-acyl glycinates from fatty alkyl esters could be overcome. An ideal process would give high yields of low-color N-acyl amino acid salts with a reduced proportion of fatty acid soaps and dipeptide by-products.